The invention relates to the preparation of phenolic resins prepared with the aid of metal salts which catalyze the formation of resols and whose cations can easily be precipitated as low-solubility salts in industrial processed.
Resols of this type are preferred for various areas of application owing to their better moisture resistance.
Examples of basic salts of this type are basic alkaline earth metal salts, in particular calcium hydroxides. These salts have the advantage over the alkali metal hydroxides usually employed that the resins prepared in this way have a very low content of free phenol and that the products produced from these resins have better moisture resistance. After the condensation, the salts can be precipitated from the aqueous solution by addition of dilute sulphuric acid, carbon dioxide, ammonium sulphate, ammonium phosphates or ammonium carbonates and separated off. This gives resins which are virtually ash-free.
It is a disadvantage of these resins that the insoluble precipitates formed in their preparation must be filtered off in a complex process, since otherwise blockage of the nozzles can occur during processing of the resins by spraying and can thus result in problems during processing.
Filtration of the resins is firstly complex and secondly associated with considerable loss of resins. In addition, disposal of filter cakes containing phenolic resins to landfill is expensive and represents an environmental problem.
A solution to these problems are phenolic resins in which calcium ions are bound to the resins in the form of complexes with the aid of ammonia or amines, as disclosed in EP-A 0 198 130 and EP-A 0 190 468. However, these complexes have only moderate stability. Within a few hours, precipitates form in the corresponding resins solution, and consequently these can only be employed if they are processed further immediately after the complexing. This is generally not the case, since the resin manufacturer is not the processor.
In addition, it has been found that calcium carbonate present in technical-grade calcium hydroxide likewise interferes with the processing of these resins and promotes further precipitation.
It is therefore an object of the invention to provide resols which are simple to prepare and which give products having good moisture resistance. It is a further object of the invention that these resols have no interfering precipitations no neutralization with the usual neutralizing agents used, such as sulphuric acid, carbon dioxide, ammonium, sulphate, ammonium phosphates or ammonium carbonate, and in which impurities of industrial products, such as, for example, calcium carbonate, have no interfering effects.
The object it achieved by a process according to claims 1 to 6. The resins prepared using the process according to the invention are used, in particular, as binders for the production of acoustic and thermal insulating materials, wood materials, foams and laminates according to, claims 7 to 10.
In phenolic resins condensed with the aid of alkaline earth metal ions, it has been attempted to keep the alkaline earth metal ions in solution with the aid of complexing agents in such a way that a good shelf life is achieved. This was not successful. As a corresponding comparative example shows, addition of conventional complexing agents to a resin solution prepared with the aid of alkaline earth metal ions did not results in the desired effect (if complexing of the metal ions.
EP-A 890 613 teaches to mix a urea-modified phenolic resin which has been condensed with the aid of alkaline earth metal hydroxide with a chelating agent and a polymeric anionic dispersant after the condensation reaction in a single working step. On repeating this teaching (Example 7), however, it was found that a precipitate is nevertheless formed and must be filtered off. In addition, the products obtained have a dark colour which restricts their use in all cases where a pale hue is desired, such as, for example, on use in the mineral wool industry.
Surprisingly, however, it has been found that mixing of the resin solution of a phenolic resin prepared with the aid of alkaline, earth metal hydroxide with a commercially available complexing agent allows complexing of the metal ions to be achieved if the resin solution has already been intensively mixed with a dispersant. The resin here is clearly in dispersed form. The resin solution formed in this way is stable. No precipitate forms. Even on contact with CO2 or even on passing CO2 into corresponding solutions, no alkaline earth metal carbonate precipitates. On the contrary, alkaline earth metal carbonate particles present in the resin solution dissolved within a short time.
It is furthermore surprisingly that the metal ions in the resins treated in this way are immobilized in such a way that they are apparently no longer available for later reactions with water. The products produced using the resins prepared in accordance with the invention are distinguished by very good moisture resistance.
The condensation reaction of phenolic compounds is carried out in the manner known per se for the preparation of resols by heating the reactants in aqueous solution until the desired degree of condensation has been achieved. The condensation partners employed can be any phenolic compounds and aldehydes known for the preparation of phenolic resins.
Phenolic compounds are, in particular, phenol, but also aliphatically or aromatically substituted phenols, and polyhydric phenols. Examples of these are cresols, xylenols, tert-octylphenol, naphthols, p-phenylphenol, bisphenols and resorcinols, but also natural products, such as, for example, cardenol, cardol and tannin. The phenolic compounds can be employed as individual compounds or in any desired mixtures with one another.
Aldehydes which can he employed are all compounds of the general formula Rxe2x80x94CHO. Examples are formaldehyde, acetaldehyde, propionaldehyde, n-butyraldehyde and isobutyraldehyde, glyoxal and furfural. The preferred aldehyde is formaldehyde, which is employed as such or in the form of a formaldehyde-eliminating substance, such as, for example, paraformaldehyde or trioxane. The preferred form of addition is an aqueous solution having a formaldehyde content of greater than 30% (formalin). The molar ratio between phenolic compound and aldehyde can be selected as desired in the range from 1:1.2 to 1:4.
Catalysts which can be employed in the process according to the invention are metal salts which catalyze the formation of resole and whose cations can easily be precipitated as low-solubility salts in industrial processes. Examples are magnesium oxide, magnesium hydroxide, the hydroxides of the alkaline earth metals calcium, strontium and barium, and salts thereof with weak acids, but also weakly basic salts of the transition metals, such as, for example, zinc acetate and manganese octanoate. The preferred catalyst is calcium hydroxide.
The amount of catalyst employed is in the range from 1 to 15% by weight preferably in the range from 2 to 6% by weight, based on the amount of phenolic compound employed.
The dispersants employed can be any commercially available substances which are marketed as dispersants, emulsifiers, wetting agents or anti-deposition agents for aqueous systems. Examples are ammonium acrylates, phosphonium salts, polyalkoxy compounds, such as, for example, alkylarylpolyethylene glycols, salts of fatty acids, in particular of alkylarylcarboxylic acids, alkylbenzenesulphonates, alkylnaphthalenesulphonates or sulphonates of products of the condensation of naphthalene or alkylnaphthalene with formaldehyde, alkyl sulphates or betaines. Preference is given to ammonium polyacrylates or alkylnaphthalenesulphonates.
In an amount of from 0.05 to 5%, based on the phenolic compound employed, they are added to the reaction mixture comprising phenolic compounds, aldehyde and catalyst and mixed intensively. They can be added either before, during or after the condensation reaction. In the latter case, the dispersant can be added to the reaction mixture before the neutralization or thereafter. The crucial factor in all cases is that the resin is in some type of dispersed form, with the aid of the added dispersant, before addition of the chelating agent. This generally requires intensive mixing of the reaction mixture with the dispersant for several minutes, at least 10 minutes in customary stirred reactors used in the preparation of the resins, before the complexing agent is added.
Complexing agents which can be employed are all water-soluble compounds known per se which form chelates with metal ions. Examples are ketocarboxylic acids, dimethylglyoxime, aminopolycarboxylic acids, diethylenetriaminepentaacetic acid, but in particular nirilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA).
They are preferably employed in an amount which is stoichiometrically sufficient to complex the metal ions of the catalysts employed, an excess of up to 10% preferably being selected in order to achieve a satisfactory complexing race. A further excess does no harm, but is inappropriate for economic reasons. However, addition of a slightly sub-stoichiometric amount of complexing agent also results in transparent resin solution.
The complexing agent is added after the condensation reaction is complete and after the freshly prepared resin solution has been mixed with the dispersant. The complexing agent can likewise be added to the resin solution either before or after neutralization of the reaction mixture.
If the complexing agent is added before the neutralization, precipitations do not form even on neutralization with acids which per se lead to low-solubility precipitations of the corresponding metal salts.
If the complexing agent is added after neutralization with all acid which has already led to precipitation, this precipitation dissolves within a short time, and an infinitely water-dilutable resin solution forms. Likewise, low-solubility salts present in the resin solution, which are in the form of impurities in the technical-grade qualities employed of the metal salts used as catalyst, dissolve after a short time.
These resin solutions have a shelf life of several weeks without any precipitations of insoluble metal salts appearing. They can thus be processed in common processes, accordingly also in spray processes, even after extended storage and transport times, without the fear of precipitating salts resulting in blockage of the nozzles.
The resin solutions are transparent and exhibit excellent impregnation behaviour. They can be cured in the conventional manner for standard resols i.e. they are self-curing on exposure to heat, but can also be employed in combination with curing agents known per se, in particular with acids. They can likewise, as is also known of standard resols, be combined with other, preferably water-soluble, thermosetting and thermoplastic resins. Owing to these properties, they are particularly suitable for the production of laminates and teams.
The cured resins exhibit excellent mechanical strengths and very good moisture resistance. They are therefore preferably suitable for the production of acoustic and thermal insulating materials and wood materials.